Such methods and devices are used, for example, in automotive engineering so that when an operator approaches or moves away, corresponding actions such as locking/unlocking the central locking, locking/unlocking the immobilizer, controlling the interior lighting, closing/opening the electrically controlled side window, etc. can automatically be carried out.
In order to allow increased operating convenience, a so-called zone model is proposed in DE 100 64 141 A1 so that when an operator approaches, a corresponding action is carried out depending on the distance to the vehicle or his presence within a specific zone.
Signal transmission methods that operate specifically within the microwave range, i.e. in the GHz range, could produce incorrect position finding because of multiple reflected signals—the so-called multipath propagation problem.
For example, signals of the ID transmitter reach the vehicle reflected from a wall instead of along the direct path. Incorrect signals or several signals of an ID transmitter are received accordingly from a vehicle or a base station in the vehicle. This makes it difficult or even impossible to determine a position unambiguously and correctly.
In the most unfavorable cases, such as a (screened off) ID transmitter covered by the operator's body in the (direct) direction of the vehicle, the signal reflected from a wall then leads to a stronger signal received from the base station, so that even any conceivable difference in the signals regarding their power or amplitude does not reliably determine the position of an ID transmitter.